It is a recent tendency to produce a lighter and smaller optical element such as a lens by utilizing an aspherical lens to reduce the number of lenses. If, however, it is attempted to produce an aspherical lens by conventional grinding and polishing, it requires an expensive and complex processing. A process, therefore, has been developed to produce a lens by directly processing a preform obtained from a gob or a glass block with a precision processed press. The lens obtained in this manner does not need grinding or polishing and, as a result, lenses can be produced at a low cost and within a short production period.
This method is called glass molding and study and development for this method have been actively made in recent years. In view of heat resisting property of a mold used for glass molding, a glass which can be softened at a lower temperature, specifically a glass having a glass transition temperature (Tg) of 630° C. or below, preferably 600° C. or below, has been sought.
On the other hand, owing to recent development of glass molding technique such as simulating a preform to a final aspherical shape of a glass after press molding or pressing a glass at a higher pressure, there has arisen possibility that a glass having a glass transition temperature (Tg) within a range from 630° C. to 670° C. can be used for glass molding. Development has also been made in material for the mold for glass molding and also a protection film for the mold and, as a result, a glass having a glass transition temperature exceeding 670° C. can be formed, by glass molding. These developments have obscured the border line between the optical glass for glass molding and general optical glasses.
Many types of optical glasses having optical constants in the vicinity of those of an optical glass which is an object of the present invention have been disclosed in the past.
Japanese Patent Publication No. Sho 53-42328, No. Sho 53-47368 and No. Sho-54-6242 and Japanese Patent Application Laid-open Publication No. 2002-284542 disclose high refractive index and low dispersion optical glasses comprising SiO2, B2O3 and La2O3 as principal components. The refractive index of these optical glasses, however, is still not sufficient for the optical constants which are required for an optical glass which is an object of the present invention. These optical glasses are also not sufficient in respect of chemical durability.
Japanese Patent Application Laid-open Publication No. Sho 52-129716 discloses an optical glass having a refractive index of 1.85 or over but this optical glass is not sufficient in Abbe number.
Japanese Patent Publication No. Sho 54-6241 and Japanese Patent Application Laid-open Publication No. Sho 54-90218 disclose optical glasses comprising B2O3, La2O3, Gd2O3 and WO3 as principal components and having a refractive index exceeding 1.85. These optical glasses, however, are not sufficient in stability to devitrification particularly in a region where Abbe number is 40 or over.
Japanese Patent Publication No. Sho 54-2646 discloses optical glasses having a refractive index exceeding 1.85 but these optical glasses are all insufficient in Abbe number and insufficient also in stability to devitrification.
Japanese Patent Application Laid-open Publication No. 2001-348244 discloses an optical glass having a refractive index of 1.875 or over and Abbe number of 39.5 or over. This optical glass, however, has a high liquidus temperature due mainly to insufficient amounts of Nb2O5 and Ta2O5 or a total amount of these components and hence has not sufficient capability of a large scale production. Besides, the optical glass disclosed in this publication has a high specific gravity with the result that weight of a lens made of this optical glass becomes excessively large and that flow control of the glass in a pipe is difficult in producing a glass preform by dripping the glass from the pipe made of Pt alloy.
Japanese Patent Application Laid-open Publication No. 2003-267748 discloses a high refractive index optical glass having a glass transition temperature of 630° C. or below This optical glass, however, has a high liquidus temperature due mainly to insufficient amounts of Nb2O5 and Ta2O5 or a total amount of these components and hence has not sufficient capability of a large scale production.
The above described publications also disclose examples which comprise F component. These optical glasses containing F component, however, have the disadvantage that they have high volatility of F in the melting of glass materials and this causes significant variation in the quality of the optical glass such as refractive index. These optical glasses containing F component moreover have a tendency toward deterioration in chemical durability and hence is not suitable for a large scale production.
Refractive index within a range from 1.85 to 1.90 and Abbe number within a range from 40 to 42 are often used in conventional optical glasses.
If an optical glass for press molding having optical constants within these ranges is used as an aspherical lens, correction of spherical aberration will be facilitated and an optical system of a more compact and a higher efficiency will be realized. Therefore, there is a high demand for an optical glass which has optical constants within the above described ranges and still enables a stable production as an optical glass for press molding and also for general uses.
The prior art optical glasses having optical constants within the above described ranges, however, have poor chemical durability and a high liquidus temperature and hence are not suitable for a stable commercial production.
It is, therefore, an object of the present invention to provide an optical glass which has optical constants of the above described ranges and still has a low liquidus temperature and high chemical durability.
It is another object of the invention to provide an optical glass for press molding having a low glass transition temperature.